I usually cut the harness out and toss helmets away. But the old velvet ones may have some decorating use. I'd cut out any harness or chin strap and then use them myself or give them to someone who knows that they aren't protection for riding. I don't think I'd put them on ebay or Craig's List in case someone bought them to actually use.
My first few helmets (w/elastics, no chin straps then ) got so old/ratty/faded/stinky, that they were thrown out. After the approved helmets came out, all of mine were returned to the company 'cause of a fall
This summer I found an old one at a tack sale, got it for $5 Elastic, brand new, and BRIGHT GREEN
I'm going to use it for decoration